Inhibitive pigment and method of making same.



UNITED STATES PATENT FFICE.

ALLER'I'ON s. CUSHMAN', or wAsHINeroN, msrmc'r or COLUMBIA.

INHIBITIV PIGML A E HOD OF MA IN SAME- following is 'a specif cation.

Durin the past few years many investigations ave been made into thecauses underlying the corrosion of iron and steel, and as a' result ofthese investigations it is now well known that many pigments whenapplied to the surfaces of these metals exert either a stimulati naction in inducing corrosion, or a retarding or inhibitin action. Thecauses underlying these two e cots are now well known. For instance, apigment which is precipitated from solution may by occlusion orsubsequent imperfect washing carry with it traces offree acids, orsalts, such as soluble sulfates or chloride, which freely ionize acidradicals. WVhen such pigments are applied to iron stimulation ofcorrosion occurs. Again, pigments which form galvanic couples with ironand which are also good conductors of electricity may set upelectrolysis and induce corrosion; such pigments are generally of thecarbonaceous group. On the other hand, the so-called inhibitive pigmentsare generally those which are very slightly soluble and which alsocontain either basic. (hydroxyl) groups or chromic acid groups. Thelatter are the better inhibitors of corrosion, and the use of pigmentssuch as zinc chromate, which is slightly soluble and ionizes a chromicacid radical, has given excellent results in practice for the protectionof iron and steel. For further information on tlle subject of inhibitivepigments, reference is made to Bulletin N0. 35, U. S'. Department ofAgriculture, A. S. Cushman. It is alsonow well known that the chromatepigments, such as zinc chromate, lead chromate and other metallicchromates, vary widely in their inhibitive values. Such pigments, whenprepared under acid conditions or in a manner otherwise faulty, maycontain acids or salts of such character as to neutralize their normalinhibitive effect, or even to,impart to them stimulative tendencies.

This invention relates to methods of treating pigments whereby they maybe freed from stimulative acids or salts, if such' be Specification ofLetters 2atent. application tiled December 27, 1310. S eri a1 N0.593,371.

Patented Mar. 28, 1911.

present, and also rendered positively inhibitive, or" improved as totheir .inhibitive quahties, by incorporating with the pigment particles,and particularly with the surface portion of such particles, the chromicacid radical.

To carry the method into efiect I-may proceed as follows: About 15 partsof potasslum or sodium bichromate,or other soluble salt containing thechromic acid radical, are dissolved in 100 arts of water, and to thissolution is adde mated. The proportion of pigment to solution isimmaterial, provided the pigment is thoroughly wet by 'thesolution. Thecon- Centration of the solution may also be widelyvaried. The mass isthoroughly stirred, and the chemical action may be expedited by heating.The chromate solution may be acid, neutral or alkaline, but should be asnearlyv as practicable free from strong mineral acids, such ashydrochloric or sulfuric acids or their soluble salts. After treatthepigment to be chro-' ment for a suitable time, depending upon the natureof the pi ment, the solution is eliminated by filtration or otherwise,and. the pigment dried, with or without previous washing with water.Examination of the pigment, after treatment as above, shows that thesurfaces of the particles contain the chromic acid radical. Thesuperficial character of the action may be shown inthe case of mostpigments by rubbing the treat,- ed pigment in a mortar or otherwisecrushing or abrading the particles, when the differing colors of theoriginal and modified portions of the pigment particle's will be madeapparent. Certain pigments, by sufficiently prolonged treatment, may becon verted completely into chromate, but such conversion is not as arule economical or desirable. The time of treatment will of course varyaccording to the'character of the pigment treated.

The pigments used are such as are capable of taking up at least a smallproportion of the chromic acid radical, either physically or chemically,usually chemically. Hence the pigments treated are generally basic incharacter. or contain metallic oxids, as for example zinc oXid, white orred lead, litharge, calcium carbonate, slag pigments, oxids of iron,etc.

Pigments containing in large proportions the strong mineral acids, assulfuric or hysubjected to the action of a chromate solution as abovefor a period of ten to twenty minutes is superficially converted intochromate and the resulting composite. pigment is found to possess a muchfiner color value than the original red lead in conjunction with a verymuch greater inhibitive value. The inhibitive efiect of other pigmentsis similarly improved. This increased inhibitive effect is due in partto the introduction into the constituent pigment .particles of thechromic acid radical, and in part also to the neutralization of thestrongly stimulative mineral acids which are usually present in smallproportions in pigments formed by precipitation methods. For this latterreason, it is found advantageous to treat by this method even chr0- matepigments, as lead or zinc chromat'es, inasmuch as their inhibitive valuemay thereby be increased or "their stimulative effect corrected. Themethod is also applicable to the treatment of pigments, such as theochers, etc., which are normally stimulators of corrosion, the effect insuch case being to render the pigment positively inhibitive by providingthe pigment particles with a superficial coating having inhibitivevalue. In the case of calcium carbonate, basic slags and other pigmentswhich are objectionable for use with linseed oil on account of theiraction in saponifying the oil and thereby preventing or checking theformation of linoxin, the treatment above (lescribcd presents especialadvantages, inasmuch as it not only reduces the objection ably basiccharacter of the pigment, but the emcee chromic acid radical alsodirectly aids in the formation of linoxin.

It is well known that zinc chromate, either in dry form or as a paste orin the readymixed state, has been mixed with paints for use on iron orsteel, but such procedure is quite unrelated to the object of thepresent invention, which is to impartincreased inhibitive value to thepigment particles themselves.

I claim: A v

1. The method of preparing pigments having inhibitive properties, whichconsists in subjecting a pigment substantially free from sulfates orchlorids to the action of the chromic acid radical.

2. The method of preparing pigments having inhibitive properties, whichconsists in subjecting a pigment substantially free from sulfates orchlorids to the action of the chromic acid radical and arresting thereaction before the conversion into chromate is complete.

3. The method of treating pigments to improve their inhibitive value orto impartinhibitive properties thereto, which consists in superficiallychromating pigment particles substantially free from sulfates orchlorids.

4. The herein-described inhibitive pigment, comprising particles ofpigment material substantially free from sulfates or chlorids andincompletely chromated.

5. The herein-described inhibitive pigment, comprising particles ofpigment material substantially free from sulfates or chlorids andsuperficially chromated.

(3. The herein-described inhibitive pigment, comprising particles of anoxid pigment substantially free from sulfates or chlorids andsuperficially chromated.

In testimon whereof, I affix my signature 1n presence 0 two witnesses.

. KLLnRToN s. GUSHMAN.

lVitnesses J 0s. H. BLACKWOOD, CLINTON P. TOWNSEND.

